1. Field of the Invention
The present invention relates generally to the electrochemical measurement of the concentration of a species of interest and, more particularly, to the electrochemical measurement of carbon dioxide (CO.sub.2) in a liquid or gaseous fluid which may contain both oxygen and water vapor.
2. Description of the Prior Art
Electrochemical reactions based on oxidation or reduction of metals and compounds at an electrode are highly selective because of the characteristic redox potential at which oxidation or reduction of the electroactive species occurs. Selection of the electrode material in combination with an electrolyte solution system has become very important in determining sensitivity and selectivity. This is especially important in situations where one species is sought to be determined quantitatively in the presence of other species which may exhibit similar reactions.
Carbon dioxide formerly was quantitatively detected by electrochemical means wherein the concentration of CO.sub.2 in air or other vapor mixtures was measured by reduction at a stationary electrode in an environment from which oxygen had been previously eliminated. Air, in which carbon dioxide is most frequently measured, normally contains both oxygen and water vapor. The reduction potentials of both oxygen and water vapor are lower than that of carbon dioxide. It was, therefore, necessary to eliminate these interfering components from the sample gas to prevent erroneous readings. This could be a complicated procedure.
More recently, a system has been proposed in which carbon dioxide may be measured quantitatively in the presence of oxygen. Such a system is disclosed in U.S. Pat. No. 4,377,466 issued to W. J. Albery. That system employs a dual electrochemical cell structure having first and second cells separated by a CO.sub.2 permeable membrane which are designed to be sequentially encountered by the sample gas. Both cells contain working, reference and counter electrodes. The first cell contains an aqueous electrolyte system and the second, a non-aqueous solvent/electrolyte system. The potentials of the first and second working electrodes are held constant with respect to the first and second reference electrodes respectively.
The first cell is energized with a potential sufficient to reduce any oxygen molecules in the sample but, not carbon dioxide molecules. The carbon dioxide molecules pass unaffected through the CO.sub.2 permeable membrane separating the cells. The second, non-aqueous cell is maintained at a potential level which is high enough to reduce carbon dioxide.
The circuit of the system is so configured that the current in the first cell is basically proportional to the concentration of the species reduced in the first cell and that of the second cell likewise proportional to the species reduced in that cell. This enables the determination of the concentration of both oxygen and carbon dioxide in a sample gas stream assuming that there are no interfering species.
While the system of Albery or similar systems represent a clear advance over earlier electrochemical systems for the detection of carbon dioxide, they do suffer from certain significant drawbacks which can lead to inaccuracies in results. First, inaccuracies may result, especially at low concentrations of carbon dioxide, with respect to the aqueous first cell inasmuch as a certain amount of the carbon dioxide will dissolve in the aqueous solution according to the equilibrium equation H.sub.2 O+CO.sub.2 =HCO.sub.3.sup.- +H.sup.+. It is also possible that the carbon dioxide may react to some extent with one or more other species in the aqueous solution.
In addition, interference brought about by the presence of water or water vapor in the cell environment is not eliminated. It is well documented in the literature that water present in non-aqueous solvents is reduced at about -1.5 v and beyond and therefore can be a serious interferent in CO.sub.2 detection. Dimethyl sulfoxide (DMSO), for example, disclosed in Albery, cited above, is not stable in the presence of water.
Furthermore, aqueous electrolyte solutions are subject to solvent loss by evaporation especially at higher temperatures. This, of course, reduces the temperature range over which such a cell may be operated successfully for any length of time.